os: don't let sendFile put a pipe into blocking mode

Use SyscallConn to avoid calling the Fd method in sendFile on Unix
systems, since Fd has the side effect of putting the descriptor into
blocking mode.

Fixes #28330

Change-Id: If093417a225fe44092bd2c0dbbc3937422e98c0b
Reviewed-on: https://go-review.googlesource.com/c/155137


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>